The physics of liquids in porous media gives rise to many interesting phenomena, including imbibition where a viscous fluid displaces a less viscous one. Here we discuss the theoretical and experimental progress made in recent years in this field. The emphasis is on an interfacial description, akin to the focus of a statistical physics approach. Coarse-grained equations of motion have been recently presented in the literature. These contain terms that take into account the pertinent features of imbibition: non-locality and the quenched noise that arises from the random environment, fluctuations of the fluid flow and capillary forces. The theoretical progress has highlighted the presence of intrinsic length-scales that invalidate scale invariance often assumed to be present in kinetic roughening processes such as that of a two-phase boundary in liquid penetration. Another important fact is that the macroscopic fluid flow, the kinetic roughening properties, and the effective noise in the problem are all coupled. Many possible deviations from simple scaling behaviour exist, and we outline the experimental evidence. Finally, prospects for further work, both theoretical and experimental, are discussed.

Imbibition in disordered media

The effects of hemicelluloses and lignin removal on water uptake behavior of hemp fibers.

Review of Disintegrants and the Disintegration Phenomena.

This issue reviews developments in wetting and wicking of fibrous materials, covering characterization of wetting; wetting and wicking of fibers, filaments, yarns, and fabrics; factors affecting wetting and wicking of fibrous assemblies; mathematical models of wetting and wicking; and application areas of the same.

Wetting and Wicking in Fibrous Materials

This paper describes the fabrication of microfluidic cloth-based analytical devices (μCADs) using a simple wax patterning method on cotton cloth for performing colorimetric bioassays. Commercial cotton cloth fabric is proposed as a new inexpensive, lightweight, and flexible platform for fabricating two- (2D) and three-dimensional (3D) microfluidic systems. We demonstrated that the wicking property of the cotton microfluidic channel can be improved by scouring in soda ash (Na2CO3) solution which will remove the natural surface wax and expose the underlying texture of the cellulose fiber. After this treatment, we fabricated narrow hydrophilic channels with hydrophobic barriers made from patterned wax to define the 2D microfluidic devices. The designed pattern is carved on wax-impregnated paper, and subsequently transferred to attached cotton cloth by heat treatment. To further obtain 3D microfluidic devices having multiple layers of pattern, a single layer of wax patterned cloth can be folded along a predefined folding line and subsequently pressed using mechanical force. All the fabrication steps are simple and low cost since no special equipment is required. Diagnostic application of cloth-based devices is shown by the development of simple devices that wick and distribute microvolumes of simulated body fluids along the hydrophilic channels into reaction zones to react with analytical reagents. Colorimetric detection of bovine serum albumin (BSA) in artificial urine is carried out by direct visual observation of bromophenol blue (BPB) colour change in the reaction zones. Finally, we show the flexibility of the novel microfluidic platform by conducting a similar reaction in a bent pinned μCAD.

Flexible microfluidic cloth-based analytical devices using a low-cost wax patterning technique

The fundamentals of wetting and wicking are reviewed. Wetting is the displacement of a fiber-air interface with a fiber-liquid interface. Wicking is the spontaneous flow of a liquid in a porous sub...

Wetting and Wicking

Coffee stains on textiles are mainly caused by the water-soluble and acidic colored substances in coffee. The acidic nature of coffee stain has been shown by ultraviolet and visible spectroscopy of coffee as a function of pH; ion-pair formation with a cationic surfactant and titration with Hyamine 1622 and a surfactant-specific electrode; and precipitation of the colored components in coffee with barium hydroxide as a barium salt. The permanence of coffee stains on textiles depends on the nature of the fibers. The affinity of coffee stain to fibers, indicated by resistance to detergency, increases in the order polyester<cotton<nylon. Coffee stain has little affinity to polyester fibers but adheres to cotton and even more firmly to nylon. The strong affinity to nylon and the pH dependence of staining suggest an ionic interaction of carboxyl and phenolic groups with amine end-groups in nylon. The ionic attraction is augmented by nonionic interactions that are enhanced by the oligomeric or polymeric nature of the staining substances. In accord with the dominantly acidic character of coffee stain, an alkaline medium is needed for the dislodgment of coffee stain from nylon fibers. Bleaching of coffee in solution with perborate, activated with sodiumn-nonanoyloxybenzenesulfonate, or Oxone® (DuPont, Wilmington, DE) obeys pseudo first-order kinetics. Oxone, a peroxysulfate triple salt, is a more powerful oxidant for coffee stain than sodium perborate, but its use is limited by the bleach fastness of dyes.

Erik Kissa

Papers

Wetting and Wicking

Coffee stain on textiles. Mechanisms of staining and stain removal.